Synthesis and reactivity of aluminium-transition metal complexes

This project falls within the EPSRC Physical Sciences - catalysis research area.

Aluminium is a highly abundant metal and has long been utilised in synthetic and industrial chemistry. In such applications, aluminium acts as a Lewis acid, given that the preferred state, Al(III) is electrophilic. Examples include the Friedel Crafts reaction and Zeigler Natta catalyst. In 2018, the first example of nucleophilic aluminium was reported by Hicks et al. This aluminyl complex was shown to behave as a nucleophile, reacting with electrophiles such as MeI, representing a complete reversal of typical aluminium reactivity. Examples of aluminium-transition metal complexes are known, and are formed by reacting an anionic transition metal with electrophilic Al(III) species. However, this approach is limited due to the lack of available transition metal reagents. Following the discovery of the aluminyl complex, it became possible to try the opposite approach to aliminium-transition metal bond formation, using the aluminium as the nucleophilic partner reacting with positively charged metal species. The aluminyl species was used, with a gold halide partner, to synthesis a complex featuring an aluminium-gold bond. Fascinatingly, the strong electron donating ability of the aluminyl resulted in a nucleophilic gold complex, in which gold is formally in the Au(I) oxidation state. This gold complex is unique in that gold behaves as a nucleophile. It was shown to react with CO2, which inserts into the aluminium-gold bond, with gold attacking the electrophilic carbon. In the same fashion, it also reacts with diisipropylcarbodiimide. It is the aim of this project to further explore this exciting new chemistry. The objective is to synthesise a range of complexes containing aluminium-transition metal bonds. It is intended to use the transmetallation approach with transition metal-halide precursors to synthesise late transition metal-aluminium systems. This method is highly novel, as it reverses the traditional reactivity of aluminium and will allow access to bimetallic systems that have previously been too synthetically challenging to make. From a synthetic point of view, inspiration comes from the analogous boryl (-BR2) and gallyl systems, which feature a lone pair on boron or gallium. In recent years, a multitude of boron-metal and gallium-metal species have been synthesised and so success with the aluminyl is also expected. It is predicted that the new species will also be capable of acting as nucleophiles at the transition metal centre, as with the gold example. Varying the metal partner will allow the reactivity to be tuned, and so the reactivity towards electrophiles and small molecules can be investigated. The impact of this work will be to introduce new reagents into synthesis and provide a new bimetallic system for small molecule activation, arising from differences in electronic properties of the metals. This project works very closely within the EPSRC research areas of catalysis and synthetic coordination chemistry. The potential for catalytic applications is widespread, given the reactivity of the gold species towards CO2 and carbodiimides. It is hoped that this reactivity can be expanded, with different metal partners being able to activate many small molecules and electrophiles. The potential to form nucleophiles from transition metals opens up new possibilities for catalytic systems. The ability to drastically alter the electronic properties of transition metals is extremely relevant to catalysis, which is fundamentally based on changes in oxidation state at a metal centre. This project seeks to investigate the unexplored chemistry of heterobimetallic aluminium systems, leading to unconventional reactivity and new modes of small molecule activation. The applications are largely related to catalysis and also synthetic chemistry.